In an MRI apparatus, imaging of a subject is attained by irradiating the subject stayed in a uniform static magnetic field generated by a static magnetic field magnet with electromagnetic waves to induce excitation of nuclear spins in the subject, receiving electromagnetic waves generated by the nuclear spins, i.e., magnetic resonance signals, and processing the signals. The irradiation of the electromagnetic waves and reception of the magnetic resonance signals are performed by an apparatus called RF (radio frequency) antenna or RF coil, which transmits and receives RF electromagnetic waves.
RF coils are roughly classified into two kinds of those called surface antennas or local antennas, and those called volume coils or volume antennas. The surface antennas have a round shape or plane shape, and have sensitivity near the antennas, and they are used by placing them next to the surface of the subject. On the other hand, the volume antennas are in a cylindrical shape, or a shape of two discs disposed upper and lower sides, and they have sensitivity in almost whole volume in the cylinder or between the discs, and used by placing a subject in that space.
The volume antennas in a cylindrical shape include those of birdcage type (for example, refer to Non-patent document 1), and those of TEM type (for example, refer to Patent documents 1 and 2). In these volume antennas, about 16 to 24 conductive elements of a rod shape usually called rungs (crossbars or rungs of ladder) are disposed in parallel to the center axis of the cylinder and along the side of the cylinder.
Such volume antennas in a cylindrical shape are used in an MRI apparatus of the tunnel type. In an MRI apparatus of the tunnel type, a static magnetic field magnet is disposed in a cylindrical shape to form a tunnel, a subject laid on a bed is entered into the inside of the tunnel, and imaging is performed.